For radioisotopic assays, the backgrounds are typically of 10-100 counts/min. BioFocus Inc. is developing a system which features exceptional background reduction and about a thousand fold increase in detection sensitivity of the labeled molecules, e.g. antibodies to endogenous opioid peptides, peptide precursors and processing enzymes. With the MPD (Multi Gamma Photon Detector) background counts are reduced to a few counts per day, by only recording events compatible with a coincident 'triple gamma' signature of the iodine isotope utilized as a label. Members of the triple gamma family have halflives ranging from minutes to days. As atoms they have diverse chemistries and can be incorporated into diverse molecules through standard chemistries. The halogen isotopes are also readily adducted to performed macro-molecules, such as the antigens and antibodies utilized in radioimmune assays (RIA). These system characteristics confer the following advantages. The quantity of isotope is reduced, because the background count is virtually eliminated. There is a corresponding reduction in isotope purchase and radioisotope disposal costs. Minute traces of biological molecules can be quantitated at much lower concentrate.